Cold-Rolled Flat Steel Product and Method for the Production Thereof

ABSTRACT

A cold-rolled flat steel product where Rm≧1400 MPa and A80≧5% and also a method for producing such product. The product includes, in addition to Fe and unavoidable impurities (in wt. %), 0.10-0.60% C, 0.4-2.5% Si, ≦3.0% Al, 0.4-3.0% Mn, ≦1.0% Ni, ≦2.0% Cu, ≦0.4% Mo, ≦% Cr, ≦1.5% Co, ≦0.2% Ti, ≦0.2% Nb, ≦0.5% V. The microstructure includes (in vol. %) ≧20% bainite, 10-35% residual austenite and martensite as the remainder. A slab, thin slab or a cast strip having said composition, is hot-rolled to form hot strip with a hot-rolling end temperature ≧830° C., coiled at a coiling temperature ≦560° C., cold-rolled at ≧30% reduction and heat-treated by firstly being heated to an annealing temperature ≧800° C., then being cooled at a cooling rate of ≧8° C./s to a holding temperature of 470° C. to greater than the martensite start temperature and then being held at the holding temperature until at least 20 vol. % bainite is present in the microstructure.

The invention relates to a cold-rolled flat steel product having atensile strength Rm of at least 1400 MPa and an elongation A80 of atleast 5%. Products of this type are distinguished by a very highstrength in combination with good elongation properties, and aresuitable as such in particular for the production of components formotor vehicle bodies.

The invention similarly relates to a method for producing a flat steelproduct according to the invention.

The term “flat steel product” is to be understood here as meaning steelsheets or steel strips produced by a rolling process and also sheet barsand the like separated therefrom.

Where alloy contents are stated here merely in “%”, this always means “%by weight”, unless expressly stated otherwise.

EP 1 466 024 B1 (DE 603 15 129 T2) discloses a method for producing aflat steel product which is intended to have tensile strengths ofconsiderably more than 1000 MPa. In order to achieve this, a steel meltcomprising (in % by weight) 0.0005-1% C, 0.5-10% Cu, up to 2% Mn, up to5% Si, up to 0.5% Ti, up to 0.5% Nb, up to 5% Ni, up to 2% Al and asremainder iron and impurities which are unavoidable forproduction-related reasons is produced. The melt is cast to form astrip, the thickness of which is at most 10 mm and which is cooledrapidly to a temperature of at most 1000° C. by sprinkling with water ora water-air mixture. Then, the cast strip is hot-rolled with aconventional reduction rate. The hot-rolling is ended at an endtemperature at which all of the copper is still in a solid solution inthe ferrite and/or austenite matrix. Then, the strip is subjected to astep of rapid cooling, in order to keep the copper in a supersaturatedsolid solution in the ferrite and/or austenite solution. After coilingto form a coil, a cold strip can be rolled from the hot strip thusobtained with a degree of cold-rolling amounting to 40-80%. This coldstrip is then subjected to recrystallization annealing, during which itis brought as rapidly as possible to an annealing temperature lying inthe region of 840° C. and held at said temperature, in order to bringthe greatest possible proportion of the copper present in the steel intosolution. This is followed by rapid cooling to a temperature amountingto 400-700° C., at which Cu precipitations form once again. In this way,precipitation hardening is intended to achieve the desired strengthlevel of the steel. At the same time, the copper content is intended toincrease the corrosion and embrittlement resistance of the steel throughthe formation of a protective oxide layer.

A further method for producing a cold strip of extreme strength is knownfrom U.S. Pat. No. 7,591,977 B2. According to this method, a hot stripcomprising (in % by weight) 0.1-0.25% C, 1.0-2.0% Si and 1.5-3.0% Mn isrolled with a degree of cold-rolling of 30-70% to form a cold strip,which is then subjected to a heat treatment completed in a continuouspass. In this heat treatment, the cold strip is heated, in a firstannealing step, to a first annealing temperature lying above the Ar3temperature thereof, in order to bring carbides present in the coldstrip into solution. This is followed by cooling, proceeding from thefirst annealing temperature and being effected at a cooling rate of atleast 10° C./s, to a second annealing temperature. This temperature isselected such that bainite forms in the cold strip, and typically liesin the range of 300-450° C. This second annealing step carried out toform bainite is performed until the microstructure of the cold stripconsists of bainite to an extent of at least 60% and of residualaustenite to an extent of at least 5% and also of polygonal ferrite asremainder. The aim here is for the microstructure to be bainitic to thefullest possible extent and for other microstructure constituents to bepresent at most in traces. The cold strip thus provided achieves tensilestrengths of up to 1180 MPa combined with an elongation of at least 9%and can be coated, if required, with a metallic layer affordingprotection against corrosion.

Against the background of the prior art explained above, it was anobject of the invention to provide a cold-rolled flat steel productwhich can be produced in a simple and operationally reliable manner andhas an optimized combination of a further increased strength and gooddeformability. In addition, the intention was to provide a method forproducing such a cold-rolled flat steel product.

In relation to the cold-rolled flat steel product, this object has beenachieved according to the invention by the flat steel product indicatedin claim 1.

In relation to the method, the object mentioned above is achievedaccording to the invention in that at least the working steps indicatedin claim 12 are performed to produce a cold-rolled flat steel productaccording to the invention.

Advantageous configurations of the invention are indicated in thedependent claims and will be explained in detail hereinbelow as thegeneral concept of the invention.

The cold-rolled flat steel product according to the invention isdistinguished by the fact that it comprises, in addition to iron andunavoidable impurities (in % by weight):

-   -   C: 0.10-0.60%,    -   Si: 0.4-2.5%,    -   Al: up to 3.0%,    -   Mn: 0.4-3.0%,    -   Ni: up to 1.0%,    -   Cu: up to 2.0%,    -   Mo: up to 0.4%,    -   Cr: up to 2%,    -   Co: up to 1.5%,    -   Ti: up to 0.2%,    -   Nb: up to 0.2%,    -   V: up to 0.5%.

Here, in the cold-rolled state, the microstructure of the flat steelproduct according to the invention consists of bainite to an extent ofat least 20% by volume, of residual austenite to an extent of 10-35% byvolume and of martensite as remainder, it being self-evident thattechnically unavoidable traces of other microstructure constituents maybe present in the microstructure of the flat steel product. Acold-rolled flat steel product according to the invention provided inthis way regularly achieves tensile strengths Rm of at least 1400 MPaand an elongation A80 of at least 5%. The C content of the residualaustenite is typically more than 1.0% by weight.

The method according to the invention for producing a flat steel productprovided or being composed according to the invention comprises thefollowing working steps:

-   -   providing a preliminary product in the form of a slab, thin slab        or a cast strip, which, in addition to iron and unavoidable        impurities, comprises (in % by weight) C: 0.10-0.60%, Si:        0.4-2.5%, Al: up to 3.0%, Mn: 0.4-3.0%, Ni: up to 1.0%, Cu: up        to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to        0.2%, Nb: up to 0.2%, V: up to 0.5%;    -   hot-rolling the preliminary product to form a hot strip in one        or more rolling passes, wherein the hot strip obtained has a        hot-rolling end temperature of at least 830° C. when it leaves        the last rolling pass;    -   coiling the hot strip obtained at a coiling temperature which        lies between the hot-rolling end temperature and 560° C.;    -   cold-rolling the hot strip to form a cold strip with a degree of        cold-rolling of at least 30%;    -   heat-treating the cold strip obtained, wherein, during the        course of the heat treatment, the cold strip    -   is heated to an annealing temperature amounting to at least 800°        C.,    -   is optionally held at the annealing temperature for an annealing        duration of 50-150 s,    -   is cooled proceeding from the annealing temperature at a cooling        rate amounting to at least 8° C./s to a holding temperature        which lies in a holding temperature range having an upper limit        of 470° C. and having a lower limit which is higher than the        martensite start temperature MS, from which martensite forms in        the microstructure of the cold strip, and    -   is held in the holding temperature range for a period of time        which is sufficient to form at least 20% by volume bainite in        the microstructure of the cold strip.

A steel strip according to the invention has a three-phasemicrostructure, the dominant constituent of which is bainite and whichmoreover consists of residual austenite and also of martensite asremainder. It is optimal here that the bainite proportion is at least50% by volume, in particular at least 60% by volume, and that theresidual austenite proportion lies in the range of 10-25% by volume,with the remainder of the microstructure being made up here too in eachcase by martensite. The optimum martensite proportion is at least 10% byvolume. A microstructure having such a composition brings about the bestcombination of Rm*A80 with the required tensile strength.

In addition to the main components “bainite”, “residual austenite” and“martensite”, it is possible for contents of other microstructureconstituents to be present, but the proportions of these are too low tohave an influence on the properties of the cold strip according to theinvention. The residual austenite is present in a cold strip accordingto the invention predominantly in film form with small, globular islandsof block residual austenite having a grain size of <5 μm, such that theresidual austenite has a high stability in the initial state and,associated therewith, a low tendency towards undesirable transformationinto martensite. At higher degrees of deformation, martensite is formedfrom this residual austenite (TRIP effect), and this increases theelongation at break.

Cold strip produced according to the invention regularly achievestensile strengths Rm of more than 1400 MPa, with elongations A80 whichsimilarly regularly lie above 5%. Accordingly, the quality Rm*A80 offlat steel products according to the invention is regularly above 7000MPa*%, with qualities Rm*A80 of at least 13 500 MPa*% typically beingachieved. A cold strip according to the invention as such has an optimumcombination of extreme strength and sufficient deformability.

The martensite start temperature, i.e. the temperature from whichmartensite forms in steel processed according to the invention, can becalculated on the basis of the procedure explained in the articleentitled “Thermodynamic Extrapolation and Martensite-Start-Temperatureof Substitutionally Alloyed Steels” by H. Bhadeshia, published in MetalScience 15 (1981), pages 178-180.

In the steel according to the invention, carbon delays thetransformation to ferrite/pearlite, reduces the martensite starttemperature MS and contributes to an increase in the hardness. In orderto utilize these positive effects, the C content of the flat steelproduct according to the invention can be set to at least 0.25% byweight, in particular at least 0.27% by weight or at least 0.28% byweight, it being possible for the effects achieved by the comparativelyhigh carbon content to be utilized particularly reliably when the Ccontent lies in the range of >0.25-0.5% by weight, in particular0.27-0.4% by weight or 0.28-0.4% by weight.

The strength-increasing action of copper can also be utilized in acold-rolled flat steel product according to the invention. In thisrespect, a minimum Cu content of 0.15% by weight, in particular at least0.2% by weight Cu, can be present in the flat steel product according tothe invention. Cu makes a particularly effective contribution to thestrength if it is present in the flat steel product according to theinvention in contents of at least 0.55% by weight, it being possible fornegative effects of the presence of Cu to be limited by virtue of thefact that the Cu content is limited to at most 1.5% by weight.

In the steel processed according to the invention, Mn in contents of atleast 0.4% by weight and up to 3% by weight, in particular up to 2.5% byweight, promotes the bainite formation, the Cu, Cr and Ni contents whichare optionally additionally present likewise contributing to theformation of bainite. Depending on the respective other constituents ofthe steel processed according to the invention, it can be expedient hereto limit the Mn content to at most 2% by weight or to increase theminimum Mn content to 1.5% by weight.

The optional addition of Cr can also lower the martensite starttemperature and suppress the tendency of the bainite to transform intopearlite or cementite. Moreover, in contents up to the upper limit of atmost 2% by weight as predefined according to the invention, Cr promotesthe ferritic transformation, with optional effects of the presence of Crin the cold-rolled flat steel product according to the invention arisingwhen the Cr content is limited to 1.5% by weight. The positive influenceof Cr can be utilized particularly effectively if at least 0.3% byweight Cr is present in the flat steel product according to theinvention.

The addition of Ti, V or Nb, which is likewise optional, can support theformation of a finer-grained microstructure and promote the bainitictransformation. In addition, these microalloying elements contribute toan increase in the hardness through the formation of precipitations. Thepositive effects of Ti, V and Nb can be utilized in a particularlyeffective manner in the cold-rolled flat steel product according to theinvention when the content of each of these elements lies in the rangeof 0.002-0.15% by weight, in particular does not exceed 0.1% by weight.

Si is present in a flat steel product according to the invention incontents of 0.4-2.5% by weight and brings about a considerable solidsolution solidification. In order to utilize this effect in aparticularly reliable manner, the Si content can be set to at least 1.0%by weight. Similarly, to avoid negative influences, it may be expedientto limit the Si content to at most 2% by weight.

In the steel processed according to the invention, Al can partly replacethe Si content. At the same time, Al, like Si, has a deoxidizing actionduring the steel production. For this purpose, a minimum Al content of0.01% by weight can be provided. Higher contents of Al prove to beexpedient, for example, when the addition of Al is intended to set thehardness or tensile strength of the steel to a relatively low value infavour of improved deformability.

A further function of Si and Al consists in suppressing the carbideformation in the bainite and therefore stabilizing the residualaustenite by dissolved C down to low temperatures.

The positive influences of the simultaneous presence of Al and Si canthereby be utilized particularly effectively when the Si and Al contentswithin the limits predefined according to the invention satisfy thefollowing condition: % Si+0.8% Al>1.2% by weight (where % Si: respectiveSi content in % by weight, % Al: respective Al content in % by weight).

The formation of the microstructure predefined according to theinvention can be ensured in particular by virtue of the fact that theMn, Cr, Ni, Cu and C contents of the steel processed according to theinvention and accordingly the Mn, Cr, Ni, Cu and C contents of the flatsteel product according to the invention satisfy the following condition

1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2

where % Mn denotes the respective Mn content in % by weight, % Crdenotes the respective Cr content in % by weight, % Ni denotes therespective Ni content in % by weight, % Cu denotes the respective Cucontent in % by weight and % C denotes the respective C content in % byweight.

To produce a flat steel product according to the invention, the primaryor preliminary product cast from a steel having a composition accordingto the invention is firstly brought to a temperature or held at atemperature which is sufficient to end the hot-rolling carried outproceeding from this temperature at a hot-rolling end temperature lyingin the range of 830-1000° C. After it leaves the last rolling stand usedfor the hot-rolling, the hot strip cools down on the roller tableadjoining the rolling stand in question. Subsequent to the roller table,the hot strip passes into a coiling device, in which it is wound to forma coil.

The coiling temperature has to be at least 560° C., so that a relativelysoft hot strip microstructure consisting of ferrite and pearlite isformed. A temperature profile which is optimal for this purpose arisesif the hot-rolling end temperature lies in the range of 850-950° C., inparticular in the range of 880-950° C. To this end, it is typically thecase that the preliminary product is heated to a temperature lying inthe range of 1100-1300° C. or is held at this temperature before thehot-rolling. The microstructure of the hot strip thus obtained consistsprimarily of ferrite and pearlite. The risk of grain boundary oxidationarising can be minimized by virtue of the fact that the coilingtemperature is limited to at most 750° C.

After the coiling, the hot strip is cold-rolled, it going without sayingthat the hot strip can be conventionally descaled by chemical ormechanical means before the cold-rolling.

The cold-rolling is effected with a degree of cold-rolling of at least30%, in particular at least 45%, in order to accelerate therecrystallization and transformation during the subsequent annealing. Itis generally the case that a better surface quality is also obtained byobserving a correspondingly high degree of cold-rolling. Degrees ofcold-rolling of at least 50% have proved to be particularly favourablefor this purpose.

After the cold-rolling, the cold strip obtained according to theinvention completes an annealing cycle in a continuous pass, duringwhich it is heated in a first annealing phase to a temperature of atleast 800° C., preferably at least 830° C. This first annealing phaselasts at least for such a period of time that the cold strip iscompletely austenitized. 50-150 s are typically required for this.

At the end of the first annealing phase, the product is quenched, thecooling rate being at least 8° C./s, in particular 10° C./s. The targettemperature for this quenching is a holding temperature which is at most470° C. and is higher than the martensite start temperature MS, fromwhich martensite forms in the microstructure of the cold strip. Inpractice, the range of 300-420° C., in particular 330-420° C., can beused as an indication of the range in which the holding temperature isto lie.

Proceeding from the respective holding temperature, the cold strip isheld in the holding temperature range in the second annealing phase, tobe precise until at least 20% by volume of the microstructure of thecold strip has transformed into bainite. The hold can be carried outhere as an isothermal hold at the holding temperature reached during thecooling or as a slow decrease in temperature within the holdingtemperature range.

The flat steel product produced according to the invention can be coatedin a conventional manner with a metallic protective layer. This can beeffected by hot-dip coating, for example. If annealing is requiredbefore the application of the metallic coating, the heat treatmentprovided according to the invention can be carried out in the course ofthis annealing.

The invention will be explained in more detail hereinbelow on the basisof exemplary embodiments.

Five steels S1-S5 were melted, the composition thereof being shown inTable 1.

The steel melts of corresponding composition were cast in a conventionalmanner to form a strand, from which slabs were separated. The slabs werethen heated in a similarly conventional manner to a reheatingtemperature.

The heated slabs were hot-rolled in a similarly conventional group ofhot-rolling stands to form hot strips having a thickness of 2 mm.

The hot-rolling end temperature was in the range of 830-900° C. in eachcase. The hot strips were cooled proceeding from this temperature to acoiling temperature lying above 560° C. and then coiled to form coils.

The hot strips thus obtained were descaled after the coiling andcold-rolled to form cold strip with degrees of cold-rolling of 50% afterthe descaling.

A relatively large number of specimens of these cold strips were thensubjected to a heat treatment, in which they were heated in a firstannealing step at a heating rate of at least 1.9° C./s to a firstannealing temperature in the range of 830-850° C. The cold strips wereheld at this temperature for a period of time of 120 s, until they hadbeen completely heated through.

This was followed by quenching, during which cold strips were quenchedat a cooling rate amounting to at least 8° C./s to a holding temperatureT2 in the range of 350-420° C. Specifically, the holding temperatures T2in a first batch of tests were 300° C., 310° C., 330° C., 340° C., 375°C., 390° C. and 410° C. The cold strip specimens were held at therespective holding temperature T2 for an annealing duration t2.

In FIG. 1, the tensile strengths Rm achieved are plotted against therespective annealing temperature T2. It can be seen that the cold stripspecimens produced from the steel S5 each achieve the required minimumtensile strength of 1400 MPa only under certain annealing conditions,whereas the tensile strengths of the cold strip specimens produced fromthe other steels were always reliably above the minimum limit of 1400MPa. The comparatively low carbon content of the steel S5, lying at thelower limit of the content range predefined according to the invention,has been identified as the reason for this.

In FIG. 2, the tensile strengths of the cold strip specimens producedfrom the steel S4 are plotted against the annealing duration t2 of thesecond annealing stage. It can be seen that the cold strip specimensheld at a holding temperature of 310° C., 330° C. and 350° C., i.e. inthe holding temperature range of 310-350° C., achieved the requiredtensile strength Rm of 1400 MPa, irrespective of the respectiveannealing duration t2.

In FIG. 3, the tensile strengths of the cold strip specimens producedfrom the steel S5 are similarly plotted against the annealing durationt2 of the second annealing stage. It can be seen here that the coldstrip specimens held at a holding temperature of 350° C. and 390° C.,i.e. in the holding temperature range of 350-390° C., achieve therequired tensile strength Rm of 1400 MPa if the annealing duration t2 isshorter than 145 s.

In FIG. 4, the elongation A80 of the cold strip specimens produced fromthe steel S4 is plotted against the annealing duration t2 of the secondannealing stage. The cold strip specimens held at a holding temperatureof 310° C., 330° C. and 350° C., i.e. in the holding temperature rangeof 310-350° C., achieved the required minimum elongation A80,irrespective of the respective annealing duration t2.

In FIG. 5, the elongation A80 of the cold strip specimens produced fromthe steel S5 is plotted against the annealing duration t2 of the secondannealing stage. Here, too, it can be seen that the cold strip specimensachieve the required elongation A80 of at least 5% irrespective of therespective holding temperature T2 thereof and irrespective of therespective annealing duration t2. Accordingly, if a short annealingduration and suitably low holding temperatures T2 are observed, it isalso possible for a cold-rolled flat steel product according to theinvention in which a high tensile strength Rm is combined with asufficient elongation A80 to be produced from the steel S5, in spite ofthe comparatively low C content thereof.

FIG. 6 shows, in a section, a magnified view of a cross section of acold strip according to the invention. In this figure, by way ofexample, residual austenite blocks RA-b are marked and a point at whichfilm-like residual austenite RA-f is present in a lamellarstratification is emphasized by being circled.

TABLE 1 Steel C Mn Si Cu Cr Ti Nb V Al N Other S1 0.52 1.48 0.40 1.510.88 0.009 — 0.093 1.400 — — S2 0.301 1.41 1.46 1.47 0.87 0.014 0.0050.09 0.021 0.0015 Ni: 0.021 Mo: <0.002 S3 0.505 1.50 0.40 0.6 1.30 0.011— 0.098 0.012 0.002  Ni: 0.63 Mo: 0.30 S4 0.384 1.97 0.41 0.57 1.370.0016 — <0.0005 0.018 0.0014 Ni: 0.59 Mo: 0.30 S5 0.252 1.47 2.15 0.320.41 0.020 — 0.11 0.009 — Ni: 0.02 Mo: <0.002 Amounts, in % by weightRemainder iron and unavoidable impurities

1. A cold-rolled flat steel product, having a tensile strength Rm of atleast 1400 MPa and an elongation A80 of at least 5% and comprising, inaddition to iron and unavoidable impurities (in % by weight): C:0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0%, Mn: 0.4-3.0%, Ni: up to 1.0%,Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to0.2%, Nb: up to 0.2%, and V: up to 0.5%, wherein the microstructure ofthe flat steel product consists of bainite to an extent of at least 20%by volume, of residual austenite to an extent of 10-35% by volume and ofmartensite as the remainder.
 2. The flat steel product according toclaim 1, wherein the C content thereof is at least 0.25% by weight. 3.The flat steel product according to either of claim 1, wherein the Ccontent thereof is at least 0.27% by weight.
 4. The flat steel productaccording to claim 1, wherein the Si content thereof is at least 1.0% byweight.
 5. The flat steel product according to claim 1, wherein the Alcontent thereof is at least 0.01% by weight.
 6. The flat steel productaccording to claim 1, wherein the Cu content thereof is at least 0.2% byweight.
 7. The flat steel product according to claim 5, wherein the Cucontent thereof is at least 0.55% by weight.
 8. The flat steel productaccording to claim 1, wherein the Cr content thereof is at least 0.3% byweight.
 9. The flat steel product according to claim 1, wherein the Mn,Cr, Ni, Cu and C contents thereof satisfy the following condition:1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2, where % Mn:respective Mn content in % by weight, % Cr: respective Cr content in %by weight, % Ni: respective Ni content in % by weight, % Cu: respectiveCu content in % by weight, % C: respective C content in % by weight. 10.The flat steel product according to claim 1, wherein the microstructurethereof comprises at least 50% by volume bainite.
 11. The flat steelproduct according to claim 1, wherein the microstructure thereofcomprises 10-25% by volume residual austenite.
 12. A method forproducing a flat steel product, said method comprising the followingwork steps: providing a preliminary product in the form of a slab, thinslab or a cast strip, which, in addition to iron and unavoidableimpurities, comprises (in % by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al:up to 3.0%, Mn: 0.4-3.0%, Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V:up to 0.5%; hot-rolling the preliminary product to form a hot strip inone or more rolling passes, wherein the hot strip obtained has ahot-rolling end temperature of at least 830° C. when it leaves the lastrolling pass; coiling the hot strip obtained at a coiling temperaturewhich lies between the hot-rolling end temperature and 560° C.;cold-rolling the hot strip to form a cold strip with a degree ofcold-rolling of at least 30%; heat-treating the cold strip obtained,wherein, during the course of the heat treatment, the cold strip isheated to an annealing temperature amounting to at least 800° C., iscooled proceeding from the annealing temperature at a cooling rateamounting to at least 8° C./s to a holding temperature which lies in aholding temperature range having an upper limit of 470° C. and having alower limit which is higher than the martensite start temperature (MS),from which martensite forms in the microstructure of the cold strip, andis held at the holding temperature for a period of time which issufficient to form at least 20% by volume bainite in the microstructureof the cold strip.
 13. The method according to claim 12, wherein thehot-rolling end temperature is 850-950° C.
 14. The method according toclaim 12, wherein the holding temperature is 300-420° C.
 15. The methodaccording to claim 12, wherein the cold strip is coated with a metallicprotective layer after the heat treatment.